Seismic surveys are performed in a variety of environments to gain a better understanding of the geometry and seismic wavespeeds of subterranean geological formations and structures. Gravity measurements also are made to provide complementary knowledge with respect to the distribution of mass in subterranean regions. Examples of gravity measurements include ship borne dynamic gravity measurements which may be made using, for example, an upgraded LaCoste & Romberg gravity meter. Ship borne gravimeters are normally mounted on gyro-stabilized platforms to minimize pitch and roll, and gravity signal outputs are heavily filtered to remove accelerations due to waves. In current practice, accurate vessel speed and direction measurements may be obtained from GPS and used to correct for gravimeter motion leading to the Eötvös correction for Coriolis acceleration, proportional to the eastward velocity component of the gravimeter. Large amplitude accelerations due to ocean waves have a dominant period of 5-10 s, and low-pass filtering below 3 minutes results in a residual ocean wave signal of less than 1 mGal. At periods longer than 1 minute, the Eötvös effect is the strongest perturbation but can be corrected accurately due to rapid sampling of navigation data at 1 s periods. Another ship borne gravity measurement method is Sea-Air Gravity Enhanced (SAGE) which is an enhanced marine inertial navigator system, WSN-7, based on a ring laser gyroscope. The ship borne systems use single gravity meters, although a vessel may operate two WSN-7 systems independently for redundancy. Current dynamic gravity measurements have a precision of about 0.2 mGal at a minimum wavelength of 0.5 km, where the spatial wavelength is determined by the filter applied to remove short period ocean wave accelerations. Additionally, sensors have been constructed to measure seismic and gravity data simultaneously. However, dynamic ship borne gravimeters remain limited to the precision described above.
Gravity gradiometry is a technique in which gradients of a gravity field are measured. The gravity gradiometry technique was initiated to improve spatial sensitivity to more local variations in mass density, and gravity gradiometers have been used in locating boundaries between density contrasts in the earth such as those due to salt bodies. More recently, dedicated equipment has been developed which detects differences of acceleration between sensors mounted on the diameter of a rotating disc. Such a sensor arrangement, in principle, allows separation of linear accelerations of the platform from the gradio-gravimetry signal, greatly reducing the sensitivity to both platform accelerations and the Eötvös effect. However, such instruments are expensive and have required deployment on dedicated vessels.